1. Field of the Invention
The present invention relates to a high-resolution NMR probe and, more particularly, to a high-resolution NMR probe capable of measuring solid samples at higher sensitivity than heretofore.
2. Description of Related Art
An NMR spectrometer is an instrument for analyzing the molecular structure of a sample under investigation by placing the sample in a static magnetic field, irradiating the sample with an RF signal, detecting a feeble RF signal (NMR signal) emanating from the sample, and extracting information about the molecular structure contained in the detected signal.
FIG. 1 schematically shows the structure of an NMR spectrometer. This instrument has an RF oscillator 1 producing an RF signal, which is controlled in terms of phase and amplitude by a phase controller 2 and an amplitude controller 3 and sent to a power amplifier 4.
The power amplifier 4 amplifies the RF signal to a level necessary to excite an NMR signal. The output signal from the amplifier 4 is sent via a duplexer 5 to an NMR probe 6 in which a detection coil (not shown) is placed. The detection coil directs an RF signal to a sample under investigation. After the irradiation, a feeble NMR signal emanating from the sample is detected by the detection coil (not shown) placed in the NMR probe 6. The signal is again passed through the duplexer 5 and then fed to a preamplifier 7, which, in turn, amplifies the signal to a receivable level.
A receiver 8 converts the frequency of the RF NMR signal amplified by the preamplifier 7 to an audio frequency at which the signal can be converted into digital form. Simultaneously, the receiver 8 controls the amplitude. The audio NMR signal from the receiver 8 is converted into digital form by an analog-to-digital data converter 9 and fed to a control computer 10.
The control computer 10 controls the phase controller 2 and amplitude controller 3, Fourier-transforms the NMR signal accepted in the time domain, automatically corrects the phase of the Fourier-transformed NMR signal, and then displays the NMR signal as an NMR spectrum.
Generally, NMR spectroscopy has the problem that the amounts of energies released and absorbed are much smaller than in visible light spectroscopy, UV spectroscopy, and IR spectroscopy because NMR spectroscopy is performed in the radio wave frequency range and, therefore, the detection sensitivity is quite low.
A method conventionally adopted to solve this problem is disclosed in U.S. Pat. No. 7,282,919. In particular, various components within an NMR probe (i.e., transmit-receive coil, preamplifier, tuning and matching circuit, etc.) are cooled cryogenically to remove their electrical thermal noise, for achieving higher sensitivity.
In the conventional method, liquid helium and liquid nitrogen are used in conjunction to cool the transmit-receive coil, preamplifier, tuning and matching circuit, etc. Alternatively, a multistage GM (Gifford-McMahon) refrigerator is used to cool those components. Consequently, the cooling structure is complex and expensive to fabricate.